**Core Concept**
The question is testing the knowledge of cerebrospinal fluid (CSF) rhinorrhoea, a condition characterized by the leakage of CSF through the nasal passages. CSF rhinorrhoea occurs when there is a breach in the dura mater, the outermost meningeal layer, allowing CSF to escape into the nasal cavity.

**Why the Correct Answer is Right**
The most common site for CSF rhinorrhoea is the cribriform plate, a thin, porous bone that forms part of the ethmoid bone in the anterior cranial fossa. The cribriform plate contains numerous small foramina, or holes, through which the olfactory nerves pass. The thin bone and presence of these foramina make the cribriform plate prone to fractures and breaches, leading to CSF rhinorrhoea. This is often due to trauma, such as a head injury, or a congenital defect.

**Why Each Wrong Option is Incorrect**
* **Option A:** The sphenoid sinus is not a common site for CSF rhinorrhoea, although it can be involved in more complex cases of CSF leak.
* **Option B:** The frontal sinus is not typically associated with CSF rhinorrhoea, as it is located more posteriorly and is not as prone to breaches as the cribriform plate.
* **Option D:** The mastoid air cells are not a common site for CSF rhinorrhoea, as they are located in the temporal bone and are not directly connected to the CSF space.

**Clinical Pearl / High-Yield Fact**
It is essential to suspect CSF rhinorrhoea in patients with a history of head trauma, especially if they present with a clear nasal discharge that does not respond to nasal decongestants. A high index of suspicion and prompt investigation, including imaging studies and CSF analysis, are crucial in diagnosing and managing this condition.

**Correct Answer: C. Cribriform plate. The cribriform plate is the most common site for CSF rhinorrhoea.**